Apparatus for displaying a suspended image

ABSTRACT

The apparatus comprises an object for display, a retro-reflector receiving light from the object, a beam-splitter in a path for light from the object to the retro-reflector, and a Fresnel lens (having no point of central symmetry) between the beam-splitter and the retro-reflector. The beam-splitter makes an oblique angle to the direction of propagation of the light, such that it transmits some light and reflects the rest. Light from the beam-splitter passes through the lens to the retro-reflector, which reflects it back through the lens to the beam-splitter. This retro-reflected light is reflected and/or transmitted by the beam-splitter to form a real image suspended in space. The type of Fresnel lens used in the apparatus improves viewability because unwanted reflections from the surfaces of the lens are shifted such that they are no longer visible when viewing the suspended image.

TECHNICAL FIELD

The present invention relates to an apparatus for displaying an imagesuspended in space.

BACKGROUND ART

Suspended images may be used, for example, as an aid to visualisationduring laparoscopic (keyhole) surgery. The potential benefits of keyholesurgery are, on rare occasions, undermined by complications followingcertain keyhole procedures. A reason for this is the misinterpretationof the anatomy of the patient seen via a standard TV, or video monitor,as a twodimensional (2D) image. Another problem is that the monitor onlyallows the surgeon a “look-up” instead of a “gaze-down” position. Thisresults in the surgeon no longer being able to see his hands whilelooking at the 2D image displayed on the monitor. It is thereforeadvantageous to suspend an image in space above the patient, enablingthe surgeon to work in the preferred “gaze-down” position.

Apparatus for displaying an image suspended in space, which may be usedfor keyhole surgery, is disclosed in PCT/GB98/00186 and European PatentNo. EP 0460 873 (Thorn EMI plc). The former apparatus comprises anobject for display, a retro-reflector, a beam-splitter, and a convergingoptical element. The converging optical element may comprise a Fresnellens.

A Fresnel lens generally has a micro-structured surface on one side, anda flat or smooth surface on the other, as shown in FIGS. 1 and 2. Themicro-structured surface consists of a series of grooves (22) withchanging slope angles as the distance from the optical axis increases.The grooves are arranged in concentric circles, the circles having apoint of central symmetry (26).

The use of a Fresnel lens in a suspended image system gives rise tounwanted images formed by reflections from the surfaces of the lens. Twotypes of reflections are formed: those from the grooved surface(‘floating reflections’), and those from the flat or smooth surface(‘fixed reflections’). Reflections from the grooved surface of theFresnel lens (which is normally situated facing the object to bedisplayed) give rise to a virtual image of the object. Reflections fromthe flat or smooth surface of the lens give rise to a real image (28) ofthe object. These reflected images are formed in addition to the imageof the object formed by the refraction of light from the Fresnel lens.

Suspended image systems may be used for applications in which thepresence of these reflected images is very distracting, as, for example,in keyhole surgery. Application of anti-reflective coatings to theFresnel lens can significantly reduce the visibility of these unwantedimages. However, such coatings can be very expensive and difficult toapply, particularly to the grooved surfaces of the lens. The coatingsmay also cause colour shifts in the suspended image.

An aim of the present invention is to improve the viewability of asuspended image by shifting the position of the reflected images suchthat they are no longer visible when viewing the suspended image.

DISCLOSURE OF INVENTION

According to a first aspect of the invention there is provided apparatusfor displaying a suspended image comprising: an object for display; afirst retro-reflector being arranged to receive light from the object; abeam-splitter in a path for light from the object to the firstretro-reflector, the beam-splitter being arranged at an oblique angle tothe direction of propagation of the light such that it transmits part ofthe light and reflects part of the light, the light being transmitted orreflected by the beam-splitter is received by the first retro-reflectorand is reflected by it back to the beam-splitter, which either reflectsor transmits part of this retro-reflected light, to form a real imagesuspended in space; a converging optical element arranged between thebeam-splitter and the first retro-reflector such that in use light fromthe beam-splitter passes through the converging optical element to thefirst retro-reflector, which reflects it back through the opticalelement towards the beam-splitter, characterised in that the convergingoptical element comprises a Fresnel lens having a substantially flat orsmooth surface and a grooved surface, the arrangement of grooves of thegrooved surface having no point of central symmetry.

A further retro-reflector may be provided to receive light beingreflected by the beam-splitter from the object, such that part of thelight being reflected from both retro-reflectors forms the real imagesuspended in space.

A further Fresnel lens may be provided in the path of light from thebeam-splitter to the further retro-reflector. Preferably the Fresnellens or lenses is/are located such that they form an image of the givenobject or image at the retro-reflector(s).

The Fresnel lens(es) may comprise an array of smaller Fresnel lenses,each smaller Fresnel lens having no point of central symmetry. The arrayof smaller Fresnel lenses collects the light which would have beencollected by the single Fresnel lens. The individual lenses of the arraymay all face the beam-splitter so that they share a common focal plane.The focal length of such “tiled” lenses can be made very short—of theorder of one or two centimeters—which enables the lens and theretro-reflector to be placed close together. This allows the apparatusto be more compact. Such lens arrays combine good quality opticalperformance, a large area over which to gather light, and a short focallength compared with the lateral dimension of the light gathering area.

Preferably at least one surface of the Fresnel lens (or lenses) iscoated with an anti-reflective coating in order to render the fixedreflection of the object substantially invisible to a viewer.

The object to be displayed may be an image being displayed by a visualdisplay unit. Alternatively, an illuminated solid object may be used inplace of the visual display unit.

The retro-reflector(s) may be composed of an array of very small, ormicro, retro-reflectors. The individual retro-reflectors may be of theorder of 100 μm. The retro-reflector(s) may comprise a sheet of beadtype retro-reflective material. Alternatively, the retro-reflectivematerial may comprise a sheet of a comer cube retro-reflector material,a triple mirror retro-reflector, or other direction selective screen.

According to a further aspect of the invention, the retro-reflector(s)may be replaced by a specularly reflective surface which forms a real orvirtual image suspended in space depending upon the distance between theobject and the converging optical element. Alternatively, a layer suchas a mirror (e.g. plane, parabolic or spherical) may be used in place ofthe retro-reflector(s). The specularly reflective surface or layer maybe partly light transmissive, or opaque, and may be curved or flat.

Preferably the beam-splitter makes an angle of between 35 and 55 degreesto the direction of propagation of light from the object to the firstretro-reflector or specularly reflective surface.

Suspended images having the best resolution are formed when the opticaldistances between the object and the converging lens, and the converginglens and the retro-reflector, are selected so that the converging lensproduces an image of the object on or at the surface of theretro-reflector. The apparatus is therefore preferably arranged suchthat the distance between the retro-reflector(s) or specularlyreflective layer and the converging optical element(s) is selectable.

BRIEF DESCRIPTION OF DRAWINGS

The invention will now be described, by way of example only, withreference to the accompanying Figures, in which:

FIG. 1 is a schematic isometric view of a Fresnel lens having a point ofcentral symmetry;

FIG. 2 is a schematic sectional view of a Fresnel lens;

FIG. 3 shows an apparatus for displaying a real image suspended in spaceaccording to the invention;

FIG. 4 shows an apparatus for displaying a real image suspended in spaceaccording to the invention;

FIG. 5 shows a schematic plan view of a Fresnel lens having no point ofcentral symmetry; and

FIG. 6 shows a schematic plan view of a further Fresnel lens having nopoint of central symmetry.

BEST MODES FOR CARRYING OUT THE INVENTION

Referring to FIG. 3 there is shown an apparatus according to theinvention for displaying a real image suspended in space. The apparatusincludes an object (10) to be displayed; a retro-reflector (12) beingarranged to receive light from the object (10), and a beam-splitter (14)in a path of light (16) from the object (10) to the retro-reflector(12). The beam-splitter (14) is arranged at an oblique angle to thedirection of propagation of the light such that it reflects part of thelight. The light reflected by the beam-splitter (14) is received by theretro-reflector (12) and is retro-reflected by it back to thebeam-splitter. Part of the retroreflected light is then transmitted bythe beam-splitter (14) to form a real image (18) suspended in space.

The apparatus further comprises a converging lens (20) arranged betweenthe beam-splitter (14) and the retro-reflector (12) such that light fromthe beam-splitter passes through the lens (20) to the retro-reflectorwhich retro-reflects it back through the lens towards the beam-splitter.

In an alternative arrangement (shown in FIG. 4), two sheets ofretro-reflective material (12, 24) are provided, and respectiveconverging lenses (20, 21) are arranged between the beam-splitter (14)and each retro-reflector (12, 24). This arrangement reduces the amountof light lost and therefore increases the apparent brightness of thesuspended image.

It is believed the reason that the presence of a converging opticalelement (20, 21) such as a lens is advantageous, is that contrary tosimple theory, a ray of light incident on a small retro-reflector willnot be returned precisely along the direction from which it came. Thisis due to diffraction effects, manufacturing tolerances, translationalshifts, or other causes of imperfect retro-reflection. As a result, theincident ray becomes a narrow cone of light after retro-reflection. Ifone places a converging lens in the path of this cone of light, thelight will behave as if it had originated at a point source, and thelens will brine the cone of light to a focus.

If the lens (20, 21) is positioned such that it forms an image of theobject (10) at the surface of the retro-reflector (12, 24), then thepoint at which the same lens will form an image of the point source fromthe cone of retro-reflected light will coincide with the position of theoriginal object (or where a beam-splitter diverts the light at theposition of the suspended image). The lens thus provides a dramaticsharpening of the suspended image (18). The improvement is such thatbead retro-reflectors (which do not retro-reflect perfectly and usuallygive rise to poor quality suspended images) can be used to formsuspended images showing resolutions of TV quality.

The converging lens (20, 21) used in the invention is a Fresnel lens,wherein the grooves (22) of the lens (20, 21) have no point of centralsymmetry. Examples of such lenses are shown in FIGS. 5 and 6, andconstitute a normal Fresnel lens cut in half, or a smaller portion ofthe lens such as a quarter, or a third. The use of such Fresnel lenses(20, 21) is advantageous as it reduces the number of unwantedreflections from the grooved surface by removing them from the line ofsight of the viewer.

What is claimed is:
 1. Apparatus for displaying an image suspended inspace, comprising: an object for display; a first retro-reflector beingarranged to receive light from the object; a beam-splitter in a path forlight from the object to the first retro-reflector, the beam-splitterbeing arranged at an oblique angle to the direction of propagation ofthe light such that it transmits part of the light and reflects part ofthe light, the light being transmitted or reflected by the beam-splitteris received by the first retro-reflector and is reflected by it back tothe beam-splitter, which either reflects or transmits part of thisretro-reflected light, to form a real image suspended in space; aconverging optical element arranged between the beam-splitter and thefirst retro-reflector such that in use light from the beam-splitterpasses through the converging optical element to the firstretro-reflector, which reflects it back through the optical elementtowards the beam-splitter, characterized in that the converging opticalelement comprises a Fresnel lens having a substantially flat or smoothsurface and a grooved surface, the arrangement of grooves of the groovedsurface having no point of central symmetry.
 2. Apparatus as claimed inclaim 1 wherein the first retro-reflector is provided to receive lightbeing reflected by the beam-splitter from the object and a furtherretro-reflector is provided to receive light being transmitted by thebeam-splitter from the object, such that part of the light beingreflected from both retro-reflectors forms the real image suspended inspace.
 3. Apparatus as claimed in claim 2 wherein a Fresnel lens isprovided in the path of light from the beam-splitter to both the firstand the further retro-reflector.
 4. Apparatus as claimed in claim 1wherein the Fresnel lens is located such that it forms an image of thegiven object or image at the retro-reflector.
 5. Apparatus as claimed inclaim 1 wherein the Fresnel lens comprises an array of Fresnel lenseseach having no central point of symmetry.
 6. Apparatus as claimed inclaim 5 wherein the array of Fresnel lenses share a common focal plane.7. Apparatus as claimed in claim 1 wherein at least one surface of theFresnel lens is coated with an anti-reflective coating.
 8. Apparatus asclaimed in claim 1 wherein the object is an image being displayed by avisual display unit.
 9. Apparatus as claimed in claim 1 wherein theretro-reflector is a bead retro-reflector.
 10. Apparatus as claimed inclaim 1 wherein the retro-reflector is a triple mirror retro-reflector.11. Apparatus as claimed in claim 1 wherein the retro-reflector is acorner-cube retro-reflector.
 12. Apparatus as claimed in claim 1 whereinthe beam-splitter makes an angle of between 35 and 55 degrees to thedirection of propagation of light from the object to the firstretro-reflector.
 13. Apparatus as claimed in claim 1 wherein thedistance between the Fresnel lens and the retro-reflector is selectable.